Capacitive liquid level sensor

ABSTRACT

A capacitance liquid level sensor uses an oscillator and sweeps the oscillator frequency at an ultrasonic rate thereby spreading the frequency spectrum and reducing interference from external radio transmitters and interference to local radio receivers.

This application contains subject matter described in copendingapplication for Capacitive Liquid Level Sensor, Ser. No. 07/466,936filed Jan. 18, 1990 and for Capacitive Liquid Interface Sensor, Ser. No.07/466,938, filed Jan. 18,1990.

BACKGROUND OF THE INVENTION

This invention relates to capacitive liquid level sensors. Such liquidlevel sensors find use in many instruments wherein a robotic probe isused to withdraw liquid from a container containing a sample to beanalyzed or a reagent.

In such robotic systems it is desirable to have knowledge of the levelof the liquid in the container such that the probe used to withdraw theliquid can be controlled to minimize contact with the contents of thecontainer. In this manner cross contamination between samples andreagents is reduced and the job of washing the probe tip is made easy.In such robotic systems the probe is introduced into the liquidcontainer and preferably maintained just below the surface of theliquid.

To accomplish this objective, various level sensors have been developed.Among those are the so-called capacitive level sensors. These are basedon the fact that any conductor exhibits a finite electrical capacitance.When the probe actually touches a liquid, the higher dielectric constantand greater surface area of the liquid results in an increased probecapacitance. These capacitance changes can be rather small so thatsensitive detection devices are required.

Devices known in the prior art that are suitable for detecting smallchanges in capacitance include bridges, RC or LC oscillators andfrequency meter counters (including heterodyning), phase locked loops,zero crossing periodometers, amplitude changes to an RC or LC filter,and phase shift changes through an RC or LC circuit.

Among the prior art capacitive liquid level sensors are Kingston U.S.Pat. No. 3,391,547 which discloses a capacitive liquid level probe for aliquid tank. He utilizes a capacitor probe, disposed in the liquid, asone leg of a bridge circuit. An unbalance in the circuit, as a result ofchange in capacitance of the probe, is detected by a phase sensitivedetector which is referenced by the fixed frequency excitationoscillator through a variable phase shifter. The variable phase shifterallows for offset adjustment.

In similar manner, Oberli U.S. Pat. No. 3,635,094, discloses acapacitive level sense means for an automatic transfer pipette. Thesample probe is utilized as the first element and a metal stand aroundthe sample vessel is the second element of a capacitor in one leg of abridge circuit. The remaining legs of the bridge consist of a variablecapacitor leg and two resistor legs. The variable capacitor leg may beadjusted such that its capacitance matches that of the probe contactingthe liquid. The bridge circuit is excited by a fixed frequencyoscillator and a differential amplifier is utilized to determine whenthe bridge is balanced indicating that the probe has contacted theliquid.

Bello et al. U.S. Pat. No. 4,326,851 discloses a level sense apparatusand method for use in an automatic clinical analyzer in which a variablecapacitor is formed by a grounded probe and a metal plate, which isconnected to the detection circuit, disposed below the sample vessel. Afixed frequency excitation signal is utilized and the capacitance changeof the metal probe resulting from the probe contacting the liquid isdetected as a voltage change in the detection circuit. This arrangementpresents a problem in that spills on the electrode or supply tray canchange the circuits operation and the circuit requires the use ofshielding pads.

Another U.S. patent, Okawa et al. U.S. Pat. No. 4,736,638 discloses aliquid level sense apparatus for use in an automatic clinical analyzer.A metal plate disposed under the sample vessel and connected to a fixedfrequency oscillator emits low frequency electromagnetic radiation upthrough the sample. The dispense probe serves as an antenna and isconnected to a detection circuit, having appropriate bandpass filtering,which detects a voltage amplitude change when the probe contacts theliquid sample. This circuit has many of the disadvantages of Bello. Inaddition, the use of low frequency limits the time response of thecircuit.

Finally, Shimizu U.S. Pat. No. 4,818,492 discloses a capacitive liquidlevel sensor for an automatic clinical analyzer. He utilizes a resistorbridge with a fixed frequency oscillator exciting one diagonal of thebridge and the probe serving as a capacitor across the other diagonal.Phase shift across the capacitor (probe), as a result of change incapacitance of the probe, is detected by a phase detector which isreferenced by the fixed frequency excitation oscillator through avariable phase shifter. The variable phase shifter allows for offsetadjustment. The output of the phase detector is filtered and comparedagainst a reference value to provide a signal indicating the presence ofliquid at the probe.

The problem with many of these prior sensors is that they tend to be runat relatively high frequencies which are up in the AM broadcast band.This causes interference problems with nearby radios because of theradiation emitting from the sensor itself. A secondary problem is thatnearby radio transmitters can seriously interfere with the level sensingand cause errors in the probe servo system.

SUMMARY OF THE INVENTION

Many of these interference problems associated with the prior artcapacitive liquid level sensors are reduced by the subject inventionwhich uses a "spread spectrum" to reduce the average energy in anyfrequency region. Thus reduces radio interference from the sensor andnarrows the width in time of the noise or beat which could occur asresult of the interference to the sensor from nearby transmitters.

According to this invention a capacitive liquid level sensor for aliquid pipetting system comprises: a pipette probe for withdrawingliquid from a sample, an oscillator coupled to the probe for applying ahigh frequency signal to the probe, the amplitude and/or phase of theoscillator being affected by the capacitance of the probe, comparatormeans for generating a level sensor signal according to the amplitude orphase of the oscillator for signaling the probe's reaching the liquidlevel of the sample, and sweep means for varying the frequency of theoscillator in a repetitive manner, whereby interference from externalradio transmitter and interference to local radio receivers is reduced.

In a preferred embodiment of this invention, the sweep frequency isgreater than about 16 kilohertz (kHz) such that it is above thefrequency components due to the step transition resulting from the probetouching the liquid. Preferably the swept frequency is varied in alinear manner (triangular wave) and the oscillator is a voltagecontrolled oscillator having a rectangular wave output. The comparatormeans may incorporate a phase detector that generates a D.C. signal thatvaries in amplitude according to the phase difference between theoscillator and the probe signal. The phase detector may be an "exclusiveOR" circuit whose output is coupled to an RC filter. In all cases, thesweep frequency is less than that of the high frequency oscillator.

According to the method of this invention, the liquid level of a liquidsample may be sensed using a probe that is used for withdrawing theliquid from the sample and comprises the steps of: applying a highfrequency electrical signal to the probe from a signal source, the phaseor amplitude of the high frequency signal being a function of thecapacitance of the probe, detecting the phase or amplitude changes inthe signal from the probe, determining when the detected changes exceeda degree corresponding to the probe reaching the liquid level of thesample, and varying the frequency of the high frequency signal from thesource in a repetitive manner at a frequency lower than that of the highfrequency signal, whereby interference from radio transmitters isreduced and radiation from the source at any given frequency is reduced.Interference from nearby transmitters will occur when the receivedsignal beats with the high frequency oscillator to give a differencesignal in the frequency range of normal sensor operation. The sweptoscillator (i.e., frequency modulation) will beat with the receivedsignal at the phase comparator to give an output "chirp", which iscapable of being filtered out. By choosing a wide sweep, the differencesignal can be made to vary over a wide frequency range, so wide, infact, that it is mostly outside the frequency range of normal sensoroperation.

The method of this invention spreads the spectrum so that the energy ina radio receiver reception bandwidth is small. More importantly, insteadof a continuous audio beat with a radio receiver, the beat is turnedinto a frequency above the audio range. This enables the system tobetter comply with governmental regulations regarding RF emissionpermitted from instruments. Furthermore, incoming RF interference (falsetriggering) is reduced.

DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention and its advantages may beunderstood in connection with the accompanying drawings in which:

FIG. 1 is a block diagram of a liquid level sensor constructed inaccordance with this invention; and

FIG. 2 is a schematic diagram of a preferred embodiment of a liquidlevel sensor constructed in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to the drawings in which FIG. 1 illustrates atypical probe 10 driven by a robotic arm 12 which is controlled by aservo drive 14 of conventional design. The probe 10, translated in theX, Y, and Z directions by the servo drive 14 of conventional design, isadapted to be moved above and introduced into any one of plural sampleor reagent containers 16.

The probe 10 is coupled through a flexible plastic tube 20 to what isdesignated as a fluid pipettor 22. The fluid pipettor 22 is able toeither expel the contents of the probe or suck through the probe thecontents of the containers 16. The tubing 20 is made of a suitablechemically inert flexible plastic such as polypropylene and the probe 10is made of a suitable chemically inert metal such as platinum orstainless steel. The robotic arm under the control of the servo drive 14is able to raise and lower the probe 10 so as to dip into and suck fluidfrom the containers 16 and also move the probe in a translationalmovement to access in X and Y directions any one of the sample, reagent,or reaction containers 16 (Only two of which are shown.).

In accordance with this invention, a high frequency oscillator 30 iscoupled to the electrically conductive probe 10 through a coaxial cable70, whose sheath is grounded. In turn the oscillator, which may be avoltage controlled oscillator (VCO), is connected to a sweep oscillator32 which preferably provides a linear (e.g., triangular or sawtooth)waveform such that the high frequency oscillator is successively sweptthrough a range of frequencies. Abrupt changes in the probe capacitance,which occur when the probe contacts a liquid, generate a spectrum offrequencies in the output of the detector. The sweep oscillatorpreferably sweeps the high frequency oscillator frequency at arepetition frequency above those frequency components generated byabrupt changes in probe capacitance. In turn, the oscillator 30,preferably is a voltage controlled oscillator, as noted, or similaroscillator whose frequency can be varied as result of an input sweepsignal.

The output of the high frequency oscillator 30 is coupled to a phasedetector 34 preferably capable of providing a D.C. output voltage. Thehigh frequency oscillator 30 is coupled through a resistor 36 to theinput of the coaxial cable 70 and a connection is made on the probe sideof the resistor 36 to the phase detector. In this manner the phasedetector is subjected to the shift in phase caused by a change in thedielectric to which the probe is subjected. In other words thedielectric is part of an RC phase shifter. There is a stray capacitancebetween the probe 10 and liquid in the container 16. The liquid providesa dielectric which is large compared to that of air. When the probetouches the liquid, the higher dielectric constant and greater surfacearea results in an increased capacitance of the probe to ground. Theoutput of the phase detector is a D.C. signal which varies in amplitudein accordance with the changing capacitance sensed by the probe.

A voltage comparator 38 compares the signal from the phase detector 34with a reference obtained by an adjustable voltage source 40. The outputof the comparator is applied to a central processing unit (CPU) 42 whichin turn is programmed to control the servo drive 14 in any conventionalmanner such as that described in U.S. Pat. No. 4,818,492. It controlsthe fluid dispenser 22 to suck liquid from the container 16 when thecomparator signals that the liquid level has been reached. Thus thecentral processing unit 42 controls both the position of the probe 10and whether the probe dispenses or sucks up fluid from a container. Suchcentral processing units are well known and will not be describedfurther since they do not relate to the particular invention which is alevel sensor.

With reference to FIG. 2, a specific circuit constructed in accordancewith the preferred embodiment of this invention for sensing liquidlevels is illustrated. In this circuit essentially two integratedcircuit chips are used. The first is phase-locked loop which may use,for example, a CD4046BM chip made by National Semiconductor. In additiona quad operational amplifier chip made by Texas Instrument Company,TLC274CN may be used. The phase-lock loop integrated circuit isdesignated by the dashed block 50. Similarly, the quad operationalamplifier integrated circuit is designated by the dashed block 52. Thephase-locked loop includes a voltage controlled oscillator 54 andseveral phase comparators only one of which 56 is shown. The voltagecontrolled oscillator 54 has several external components which have beenselected to provide a nominal high frequency of 1 MHz, i.e., by choiceof resistors R1 and R2 and capacitor C1. The selection of these valuesis described in the application notes for the chip from NationalSemiconductor. Furthermore, the resistors R1 and R2 have beenproportioned such that the VCO input will sweep the oscillator frequency200 kHz at an approximately 20 kHz rate.

The frequency of VCO 54 is caused to change by a sweep oscillator in theform of an astable oscillator which is constructed as part of the quadoperational amplifier chip 52. The sweep oscillator, designated 58, isconstructed such that the output is applied through resistor R7 andcapacitor C6 to the inverting input of the amplifier labelled Q2.Further, the output of Q2 is applied through resistors R8 and R9 to thenoninverting input of the amplifier. Its operation is understood bysupposing that the output of the amplifier goes high. The voltage at thenoninverting input will go high. The voltage at the inverting input willremain low because of capacitor C6. As charge accumulates on capacitorC6 a time will come when its voltage exceeds that of the noninvertinginput, at which time the output of Q2 will swing low. In a similarfashion resistors R8 and R9 apply a low voltage to the noninvertinginput of Q2. Because of capacitor C6 the voltage at the inverting inputwill remain high. This status will remain until the voltage across C6 isdischarged to a voltage below that of the noninverting input at whichtime the output of Q2 will swing high and the cycle will repeatendlessly.

In this circuit it is customary to take the voltage from the outputwhich is a square wave 62. However to obtain a voltage to provide alinear sweep of frequency of the oscillator, a sawtooth or triangularwaveform is preferred. This is the signal found at the junction of R7and C6. This approximately triangle wave 60 is applied to the VCO 54input. This signal causes the voltage controlled oscillator to sweepapproximately 200 kHz around the nominal 1 MHz frequency. The rate atwhich it sweeps up and down is approximately 20 kHz and is determined bythe values of the resistors R₇, R₈, R₉, and C6.

The output of the voltage controlled oscillator 54 is designated by thesquare waveform 64. The output of the VCO is applied to two portions.One portion is supplied to a phase comparator 56. This serves as thereference signal and is illustrated by the waveform 66. The otherportion of the output of the VCO is supplied to an RC phase shiftercomposed of elements R3, C2 and the probe. Capacitor C2 is used as theD.C. blocking capacitor. The actual capacitance affecting the phaseshift is comprised of the capacitance of the coaxial cable labelled 70and the capacitances to ground of the probe. The probe is metal asdescribed. It may have plastic tubing which is attached to a pump (notshown). At the junction between R3 and C2 is a signal labelled 76 thatis affected by the dielectric of the sample whose level is sought. Thissignal 76 is an approximate triangle wave and is applied to the signalinput of phase comparator 56.

Phase comparator 56 is of the "exclusive OR" variety. The output of thephase comparator is a series of pulses, the width of which depends onthe phase difference between the reference signal 66 and the inputsignal 76. The output of the phase comparator 56, in the form of a pulsetrain 78, is applied to an RC filter network 84 composed of resistor R4and capacitor C3. The purpose of this filter is to remove the pulsesfrom the phase comparator output and produce an approximate D.C. levelproportionate to the area of the waveform 78. If the pulse width of 78changes then the approximate D.C. level of the filter 84 will change.The changing D.C. level is represented by the waveform 80 which isapplied to a differentiator 90, the heart of which is an operationalamplifier Q1, a member of the quad operational amplifier 52. Thus, toeffect the differentiation, the output of the RC filter 84 is appliedthrough resistor R5 and capacitor C4 to the input of the amplifier 90.The feedback portion of the amplifier 90 is composed of R6 and C5 inparallel. These components have been selected to form a differentiatorfor low frequencies, namely the changing portion of waveform 80. Thesecomponents also filter out high frequency noise that might leak throughthe filter network 84.

The output of the differentiator 90 is in the form of pulses, the heightof which is dependent on the rate of change and extent of change ofwaveform 80. This output signal is represented by the waveform 82. Thesepulses can then be discriminated with a window comparator to selectpulses of sufficient amplitude to represent a meaningful transition inthe capacitances at the probe which, of course, is sensitive to thedielectric effect of the sample level. The window comparator is composedof amplifiers of operational amplifiers 52 labelled 94 and 96. In theseamplifiers the signal level is compared against the voltage labelled V1and V2. For example, if the input voltage to 94 is applied to theinverting input, whenever the input voltage is below V1 the output willbe high. For the period of time that the input voltage rises above V1,the output will remain low. Thus, the positive going pulse in waveform82 causes a negative going pulse in waveform 98. In a similar fashionthe negative going pulse in waveform 82 appears as a negative goingpulse from circuit 96 and has a waveform labelled 100. The two waveforms98 and 100 are the outputs of the circuit. Waveform 98 has a negativegoing pulse whenever the probe encounters an increase in capacitance aswhen it touches a liquid. Waveform 98 has a negative going pulsewhenever the probe encounters a liquid of increased conductance ordielectric constant. In a similar fashion waveform 100 is a negativegoing pulse whenever the probe decreases in capacitance. For example,when the probe is withdrawn from a fluid of high dielectric orconductance properties.

The frequency modulation that is applied to the VCO 54 can result in aperturbation of the detecting means. Care in the selection of the sweepfrequency, its amplitude and the band pass of the filter 84 will reduceor eliminate any side effects. For example, some of the FM will resultin an output from the phase detector, since the phase shift of an RCwill vary with the applied frequency, so the output of the phase shiftdetector 84 will have a component of frequency corresponding to thesweep modulation and amplitude corresponding to the phase shift throughthe probe RC circuit. The filter on the output of the phase detector iseasily selected to roll off and reject the interfering frequencies.Furthermore, the differentiator is designed to also reject the sweepfrequency. The combined result is a detector signal that is free fromthe effects caused by frequency modulation of the oscillator.

Selection of the sweep frequency is constrained by the desire to raiseit so simple filtering will reject it. However the frequency sidebandsof the high frequency oscillator will be multiples of the fundamentalsweep frequency. If the sweep frequency is too high and the depth ofmodulation is too low, the RF energy will be concentrated in a fewdiscrete frequencies and will not achieve substantial energy spread toreduce interference.

While the preferred embodiment of the invention described uses a phasedetector and RC phase shift circuit it is to be understood that any ofthe devices known in the prior art using a source of oscillations fordetecting small changes in capacitance at the probe may be used. In eachcase, the high frequency oscillator is subjected to a sweep frequencythat phase or frequency modulates its frequency. The high signaloscillator's frequency is affected in some manner by the probe'scapacitance and the affection is detected whether it be bridgeunbalance, amplitude change in an RC or LC filter, phase shift in an RCor LC circuit or whatever. The signal 76 affection depicts a change inamplitude and/or phase over that of waveform 64, the degree of changebeing a function of the sample dielectric. The point of the invention isthe reduction in an interference as described by the use of the sweptfrequency oscillator.

As explained, the use of this invention reduces the interference of thecapacitance liquid level sensors with surrounding AM radios. Furthermorelocal RF transmitters which can otherwise seriously interfere with thefunction of the level sensor have reduced effect. The invention spreadsthe spectrum of the high frequency oscillator signal so as to reduce theenergy in any frequency region, thus reducing AM radio interference.Further the invention narrows the blip resulting from the beat with aninterfering radio transmitter so that simple filtering removes thepotential interference.

What is claimed is:
 1. In a capacitive liquid level sensor for determinethe liquid level of a sample in a liquid pipetting system, having:apipette probe for withdrawing liquid from a sample, an oscillatorcoupled to the probe for applying a high frequency signal to the probe,the amplitude and/or phase of the signal being affected by thecapacitance of the probe, and comparator means for generating a levelsensor signal according to the amplitude or phase of the high frequencysignal for signaling the probe's reaching the liquid level of thesample, the improvement comprising: sweep means for varying thefrequency of the oscillator in a repetitive manner, at a predeterminedsweep frequency whereby interference from external radio transmitter andinterference to local radio receivers is reduced.
 2. The liquid levelsensor according to claim 1 wherein the sweep frequency is greater than16 kHz.
 3. The liquid level sensor of claim 2 wherein the sweep meanshas a frequency lower than that of the oscillator.
 4. The liquid levelsensor of claim 2 wherein the oscillator frequency is varied linearly bythe sweep means.
 5. The liquid level sensor of claim 4 wherein the sweepmeans has a frequency lower than that of the oscillator.
 6. The liquidlevel sensor of claim 1 wherein the oscillator frequency is variedlinearly by the sweep means.
 7. The liquid level sensor of claim 6wherein the sweep means has a frequency lower than that of theoscillator.
 8. The liquid level sensor of claim 1 wherein the comparatormeans is a phase detector.
 9. The liquid level sensor of claim 8 whereinthe oscillator is a voltage controlled oscillator having a rectangularwave output and the comparator means is an exclusive OR circuit.
 10. Theliquid level sensor of claim 9 wherein the sweep means includes a sourceof a triangular voltage wave signal coupled to the voltage controlledoscillator.
 11. The liquid level sensor of claim 1 wherein the sweepmeans has a frequency lower than that of the oscillator.
 12. A method ofsensing the liquid level of a liquid sample using a probe, having acapacitance, for withdrawing the liquid from the sample, comprising thesteps of:applying a high frequency electrical signal to the probe from asignal source, the phase or amplitude of the frequency signal beingaffected by the capacitance of the probe. detecting changes in the phaseor amplitude of the high frequency signal, determining when the detectedchanges exceed an amplitude or value corresponding to the probe reachingthe liquid level of the sample, and varying the frequency of the highfrequency signal from the signal source in a repetitive manner at apredetermined sweep frequency, whereby interference from external radiotransmitters is reduced and radiation from the source at any givenfrequency is reduced.
 13. The method set forth in claim 12 wherein thesweep frequency is greater than 16 kHz.
 14. The method of claim 13wherein the signal source frequency is varied linearly during eachrepetition.
 15. The method of claim 12 wherein the sweep frequency isless than that of the high frequency signal.